Powered wheelchairs may be of different configurations, one aspect being to how the drive wheels are placed. Mid-wheel drive wheelchairs have front wheels, rear wheels, and drive wheels arranged between the front wheels and the rear wheels. The front and rear wheels may in this case be caster-type wheels. In comparison to front-wheel and rear-wheel driven wheelchairs, mid-wheel drive wheel chairs are sometimes considered to present a better manoeuvrability, mainly in terms of a lesser turning radius. In addition to manoeuvrability, some other important aspects when designing and configuring a wheelchair are stability and the ability for a wheel suspension assembly to ensure that all wheels are in contact with the ground at all times. Traditionally, there has often been a trade-off for one against the other in the context of mid-wheel drive wheelchairs. Stability may in one aspect be defined as the ability of preventing tipping of the wheelchair about its pitch axis, e.g. tipping over in the forward or rearward direction or lifting of rear or front caster wheels when driving on a ramp or a slope. It is especially important that the drive wheel maintains traction against the ground since the wheelchair otherwise will lose its ability to be driven, in any direction. The risk for loss of tractive force increases when operating the wheelchair on a non-planar surface, e.g. on uneven ground. The phenomena when such a situation occurs for a mid-wheel drive wheelchair is sometimes referred to as high-centering, meaning that at least one of the drive wheels has lost contact with the ground. There is also an increased risk of losing traction when entering or leaving a slope, downhill or uphill.
The suspension assembly of a wheelchair generally comprises at least one shock absorber, typically comprising a spring and a damper, swing arms (a.k.a. link arms or pivot arms) pivotably connected to the chassis to which front and rear caster wheels are mounted, the wheels themselves and the connections of swing arms and/or wheels to the chassis. The shock absorber is generally arranged between the chassis and at least one swing arm. Compression springs may generally be divided into five different types depending on their compression behavior, i.e. the rate at which the spring compresses. The different types are: linear, progressive, progressive with a knee, almost constant and degressive. Previous solutions have typically used springs of either linear or any of the progressive types listed above. All of these spring types provide an increase in spring force the greater the movement of the swing arms. This has typically also led to the suspension as a whole exhibiting the same characteristic.
U.S. Pat. No. 7,896,394 B2 discloses a mid-wheel drive wheelchair with independent front and rear suspension to enable a better ability to ascend and descend obstacles. The wheelchair includes a frame, and a front pivot arm pivotally mounted to the frame at a front pivot point, the front pivot arm having a caster wheel. A rear pivot arm is pivotally mounted to the frame at a rear pivot point, the rear pivot arm having a caster wheel. A ground engaging centre-placed drive wheel is connected to the frame between the front pivot caster wheel and the rear pivot caster wheel. A linkage connects the front and rear pivot arms to each other in a manner such that an upward or downward rotation of one of the pivot arms about its pivot point causes rotation of the other pivot arm about its pivot point in an opposite rotational direction.
U.S. Pat. No. 8,851,214B2 also discloses a mid-wheel drive wheelchair with a suspension arrangement which comprises a linkage connecting front and rear link arms.